The present invention relates in general to boiler tube supports, and in particular to a new and useful arrangement for attaching adjacent tubes to each other without the use of dissimilar metal welding.
Support lug castings are often used to support and/or transfer structural loads between adjacent boiler tubes. A typical installation of a traditional support lug casting arrangement that involves welding support lug castings to the tubes (a male lug casting to one tube and a female lug casting to an adjacent tube) is shown schematically in FIGS. 1 to 5. During boiler operation, flow through the tubes generally keeps the tube metal temperature at 1050.degree. F. or less. However, the lug castings welded to the tubes to support the tubes have limited cooling by this flow and thus operate at a higher temperature. Thus, lug castings fabricated from low chromium-containing ferritic material are susceptible to creep damage and to oxidation. Higher chromium-containing ferritic stainless steel castings are not used because of the welding difficulties they present. In order to avoid or minimize lug casting creep and oxidation in this elevated temperature environment, austenitic rather than ferritic castings, are generally used because of their improved creep and oxidation resistant characteristics.
FIG. 5 is a side elevational view of two adjacent ferritic tubes 2 and 4 which are engaged with each other by a lug casting assembly generally designated 6. As best shown in FIGS. 1 to 4, a male lug casting 7 is welded to the side of tube 2 and a female lug casting 8 is welded to the side of tube 4. When assembled, an integral tab extending upwardly from casting 7 is hooked under the casting 8 so that tube 4 in effect is supported by tube 2.
The use of austenitic castings, although attractive for creep and oxidation resistant properties necessitates a dissimilar metal weld between the castings and the ferritic tubes. In certain boiler applications, this dissimilar weld attachment has failed and, in some cases, been the source of through-wall failures of the tube.
Attempts by most designers of support lug castings to eliminate these failures have focused upon changing or improving the weld materials and/or process associated with the dissimilar metal weld. No significant activity has been directed toward eliminating the dissimilar metal weld all together.
The dissimilar metal weld would be eliminated by using austenitic tubes. This alternative, however, has generally not been given much consideration because of the added cost of austenitic tubes. Another alternative to eliminate the failures associated with dissimilar weld lug castings which has been considered is to eliminate the lug castings and support the boiler tubes from above. This alternative is not attractive because of boiler space limitations, the required boiler modifications and the associated costs.